To adjust the throat length you need a throating reamer and you can only go longer than the throat that the reamer comes with. Typically when you order a custom reamer you specify what bullet your shooting and it will be throated for it or you send in a dummy round how you want it.

Most likely a 6XC reamer would be set up for VLD's since long range competition is what it was designed for. If not then they could buy a throating reamer and set the throat for VLD's since they are the longest bullets used in 6mm typically. If you want a different neck dimension then you'd have to order a new reamer or buy a necking reamer.

It's always OK to call the smith and make sure your both talking about the EXACT same specs. It will be very frustrating for both of you if the barrel has to be set back or settle for something you don't want. He will probably have a print available that can be emailed for faxed.